The present invention is related to an improved control valve arrangement for regulating fluid communication between a source of fluid under pressure and a hydraulic motor such as is used in adjusting the position of a double-acting hydraulic cylinder on earthmoving equipment. More particularly, the invention relates to such control valve arrangement having a pressure compensated flow control valve spool and relief valve combination with improved responsiveness to dynamic fluid forces acting on the spool in order to reduce to a minimum unbalanced axial forces exerted thereon.
In the control valve arrangements of U.S. Pat. No. 3,847,180 issued Nov. 12, 1974 to N. W. Kroth, et al., and U.S. Pat. No. 3,995,532 issued Dec. 7, 1976 to J. A. Junck, et al., both of which are assigned to the assignee of the present application, the pressure source or pump is in communication with an inlet chamber of the control valve and a relief valve to provide load responsive operating fluid flow and pressure when the control valve spool is conditioned to communicate fluid from the inlet chamber to the hydraulic motor or cylinder. The relief valve or dump valve is subject to variable loading such that in a neutral condition of the valve spool the fluid communicating with the inlet chamber is returned to the reservoir with the relief valve pressure setting established at a relatively low value so as to reduce heat generation and power consumption by the system. On the other hand, upon movement of the valve spool sufficient to direct fluid from the pump and the inlet chamber to a service chamber and the cylinder, the relief valve is subjected to the pressure in the service chamber so that it regulates the pressure in the inlet chamber to both the load pressure and the relief valve pressure setting as established by its biasing spring. Since the release pressure of the relief valve is thereby a function of the load pressure rather than a high fixed value, the control valve spool experiences reduced flow forces which facilitates its manipulation.
In the case of the valve arrangement of aforementioned U.S. Pat. No. 3,995,532, the relief valve is responsive to a fluid signal from the control valve for commencing modulation of the pressure in the inlet chamber when the control valve spool is moved to operate the cylinder prior to directly communicating the inlet chamber with a service chamber of the cylinder. This preconditioning of the inlet chamber pressure is a marked improvement over prior art arrangements, as it results in more responsive movement and control of the cylinder in the direction desired.
In addition to the relatively critical "timing" relationships between certain lands, passages and metering slots of the valve housing and the control valve spool which must minimize so-called "dead band" relationships and undesirable cross-flow or leakage which might result in poor control of any load on the cylinder, any arrangement of this type should also have a minimum of unbalanced axial forces acting on the spool when fluid is flowing dynamically past it. The magnitude of the axial forces acting on the spool under such conditions is related to the pressure drop urging fluid flow through the valve spool, the volume of fluid flowing past it, the flow area defined between the spool and housing, and the direction of fluid flow with respect to the spool axis. While the valve arrangement of previously discussed U.S. Pat. No. 3,995,532 has proven to be quite satisfactory, it does experience the generation of such unbalanced axial forces. Particularly, the metering-out forces generated by fluid flow radially outwardly from the spool axis and from the cylinder back to the reservoir are much greater in magnitude than the metering-in forces generated by fluid flow across the spool from the inlet chamber to the cylinder. However, since control of axial fluid flow forces in a metering-out form of construction is more difficult than in a metering-in construction, certain limitations are imposed upon the arrangement of U.S. Pat. No. 3,995,532 which result in the retention of some unbalanced forces on the valve spool.